1. Field of the Invention
This invention relates to display processors and more particularly to a video display system employing a central processing unit, a display memory, and a cathode ray tube display device.
2. Prior Art
Computer controlled video display systems, typically employing cathode ray tubes as the display device, are used as output devices in many computer systems. When the common type of low persistence cathode ray tube is used as the output device it is necessary to repeatedly generate the video command signals at a rapid rate, typically 60 frames per second. While it would be possible for a central processing unit to directly and continuously provide the video data required for this regeneration process, that arrangement would be wasteful of the computer's resources since text to be displayed is typically unchanged in most of its detail during successive repetitions. A more economical arrangement provides a memory for storing the text to be displayed and allows the central processing unit to modify the contents of that memory as required. These display memories and the associated components which generate the required display from the memory contents are termed display processors. A wide variety of such processes have been employed or proposed. Typically the display processors have employed circuitry for assembling a series of character codes representative of a single horizontal line across the display screen from the display memory and have continually scanned or recirculated this information until the plural number of successive horizontal scans required to write these lines on the display have been performed. Some systems have used recirculating delay lines to continually present the required one of the characters in each line segment to a character generator. These systems have been relatively complicated and accordingly expensive and as the cost of computation ability has decreased the relative cost of the display processor portion of the system has increased.